1. Field of the Invention
This invention relates to superplastically formed Aluminum Lithium workpieces and more particularly to a process for thermo-mechanically conditioning such workpieces so that their yield strength and other physical properties are improved by up to 10 percent over those of unconditioned references. The process presented here concerns itself with parts made of Aluminum-Lithium through conventional superplastic forming techniques, but conditioning processes specified are applicable to parts made from other fine grain Aluminum-Lithium alloys including those with solutes of copper and magnesium, provided proper modifications to temperatures, times and pressures are made.
Aluminum-Lithium sheet stock, as provided by commercial mills, is preconditioned by mill processes to provide a variety of specifications on hardness, tensile strengths and ductility. When such preconditioned stock is used for fabrication of parts though superplastic forming procedures, significant enhancement of these parameters is possible. Specifically, when such parts are thermo-mechanically conditioned by quenching, stretching and aging, they show mechanical properties and physical characteristics significantly superior to those of unconditioned parts. It should be noted that both conditioned and unconditioned Aluminum-Lithium workpieces possess mechanical properties and physical characteristics superior to those of conventional aluminum parts at weight savings of up to 10 percent and at similar increases in stiffness.
2. Description of the Prior Art
Commercially produced Aluminum-Lithium alloys contain about 3% Lithium by weight with Lithium atoms and compounds being disposed relatively uniformly throughout the aluminum matrix. Uniformity of crystalline structure in mill standard Aluminun-Lithium stock is intentionally distorted by mill processes to provided precipitation loci throughout the metal matrix, at which loci, increased resistance to laminar shear is created. A variety of atomic-molecular activity also results from these processes which produces strained lattice structures and serendipitous increases in yield strengths, certain toughness parameters and other mechanical properties. Stresses induced in the alloy result in dislocation sites where subsequent precipitation of Aluminum-Lithium compounds can occur. Conditions for optimal precipitation and associated strengthening of Aluminum-Lithium alloys are well understood and employed by those skilled in this art.
Needs of commerce and industry for lightweight, tough, high stress tolerant parts and products has lead to intense research into Aluminum-Lithium alloys, and a compendium of this technology is available in the open literature. Some examples of such are found in papers presented at the Second International Aluminum-Lithium conference of the Metallurgical Society of the American Institute of Mechanical Engineers, Conference Proceedings, Apr. 12-14, 1983 (Library of Congress #83-83124 and ISBN #0-89520-472-X).
Conventional processes used for creation of superplastically formed parts of Aluminum-Lithium utilize associated technology and, while that technology is not presented as directly applicable to the within process, it is germane to production of preforms suitable to conditioning thereby. Commercially produced Aluminum-Lithium alloys containing major alloying elements of copper or magnesium, or combinations of the same, are also suitable for use with workpieces processed per this disclosure.